14N quadrupole double resonance in 2-, 3- and 4-nitrobenzoic acids

Abstract

¹⁴ N quadrupole cross-relaxation spectra of 2-, 3- and 4-nitrobenzoic acids show weak transitions which do not appear in their level-crossing double-resonance spectra. The multiplicity of transitions in 4-nitrobenzoic acid can be understood as originating in disorder in the sense of rotation of the NO₂ group with respect to the plane of the aromatic ring. ¹⁴N spin–lattice relaxation in the 3- and 4-nitrobenzoic acids appears to be governed by torsional oscillations of the NO₂ group about the C—N bond, so that the appearance of lines from the disordered molecules in the cross-relaxation spectra could be due to their lower torsional frequencies and hence shorter relaxation times.